The Trend Variation Feature of Fog Days in Fujian Province for Recent 44 Years

The monthly data of fog (visibility less than 1000 m) and dense fog (visibility less than 500 m) from 50 meteorological stations in Fujian Province in the period of 1961—2004 are used to analyze the annual and seasonal distribution feature, the long term trend variation, the annual and decadal variation and possible influence factors etc.The results show that the regions where annual and seasonal fog occurs frequently are in the central and western Sanming, while few occurrences are in the coast and the south of Fujian, the same is dense fog.Among the more fog regions, the number of annual fog days is more than 80, some exceed 100 days.In less fog regions it is less than 20 days.More than 30%of fog days in more fog areas are dense fog, some areas such as the southern Nanping it can even pass 50%. Dense fog seldom occurs in the coast.Fog occurs frequently during autumn and winter (October to next February) in inland and in spring in the coast areas.In summer, fog events are not prone happening.The time which dense fog appears often is in October to next February.The tendency of annual and seasonal fog days significantly decreases in large parts of Fujian with the tendency coefficients above 99.9%confidence level for 30 stations, and only in western Longyan and parts of the coast the tendency increases.For dense fog, the decreasing tendency is less than that of fog, there is nearly no change in the tendency in the south and the middle coast in Fujian, only in the middle inland the tendency has significant decrease. Analysis is focused on decadal average numbers of dense fog and its deviation coefficient.It is pointed out that though the dense fog days have a small decrease in 1990s but its deviation coefficient is the biggest, therefore the dangerous of dense fog increases while the predictability is difficult.The annual and seasonal fog days represent significant decadal variability.The annual fog days are below the normal after the mid 1980s, before which it is above the normal.Special analysis is given to 6 representative stations for the tendency and monthly variable rule of fog and dense fog.Furthermore studies of the reasons for the decrease of fog point out that the fog days have good negative relation with annual average temperature, and good positive one with relative humidity.The remarkable jump points for the three are in the mid 1980s.After that time the temperature increases faster, while relative humidity reduces remarkably and the numbers of fog decrease too.It still has a certain relationship with the dimension of forest.But due to the limited data, it now has some uncertainties.